The present invention relates to an apparatus for measuring noise factor and available gain in a linear two-port circuit.
Generally, the performance of a circuit processing weak signals, e.g., a receiver is described by noise factor, available gain, and sensitivity. Of these parameters, the sensitivity can easily be calculated from a frequency band width of the receiver, if the noise factor and available gain are known. In designing and manufacturing the receiver, the measurement and evaluation of the noise factor and available gain are of very importance. The measurements of these parameters have been separately made.
The available gain is given by a ratio P2/P1, where P1 is an available signal power of a signal source and P2 is an available signal power after it is passed through a device under test (referred to as DUT), when the signal source and DUT are connected in series.
The noise factor is defined as the ratio of (1) the total noise power available at the output port of the DUT when the input termination is at 290K to (2) that portion of (1) engendered by the input termination via the primary signal channel. The noise factor represented in dB is called as a noise figure. The noise factor is measured by connecting a noise source and DUT in cascade and by turning on and off the noise source. If it is assumed that the equivalent noise temperatures of the noise source are T2 and T1 and the noise power outputs are N2 and N1 respectively when the noise source is turned on and off, the noise factor F is expressed ##EQU1## where t.sub.ex2 and t.sub.ex1 are each an excess noise ratio of the noise source, and are given by T2/To-1 and T1/To-1 when a standard temperature To=290K, and Y called a Y factor is given by N2/N1.
However, a load included in a meter for measuring a noise power produces a thermal noise. Therefore, the meter can not directly measure the noise power of the DUT itself. Of course, if the equivalent noise temperature of the meter is at absolute zero the thermal noise is not produced. But, it is not practical to keep the temperature of the meter at absolute zero during the measurement. On the other hand, if the thermal noise from the meter is much smaller than the noise power output from the DUT, it can be neglected. Where the available gain of the DUT is large, the thermal noise from the meter will be negligibly small in comparison with the noise output of the DUT. However, recent receiving devices for higher frequencies are mostly provided with solid state components such as bipolar or field effect transistors, which generally have a relatively small available gain. It is necessary to connect an amplifying stage after the DUT if the DUT is such a solid state element having a small available gain. The noise factor of only the DUT has been separately evaluated on the basis of Friis formula from the overall noise factor. Assuming that the noise factor and available gain of the first stage (DUT), and the noise factor of the second or subsequent stage are respectively F1, Ga and F2, the overall noise factor Fm is expressed EQU Fm=F1+(F2-1)/Ga (2)
From the above equation, F1 can be evaluated separately from the measured Fm, if F2 and Ga are known.
Generally, the noise factor and available gain of the DUT connected with the noise source are dependent on the source admittance Ys. The circuit characteristics at the second or subsequent stages are also dependent on the output admittance Yout of the DUT. Accordingly, the Friis formula should be written as ##EQU2##
Since Yout is dependent on the DUT, F2(Yout) must be calibrated for each DUT. The calibration of F2(Yout) is difficult, then the accuracy of the calibration is low. Additionally, the available gain Ga is measured by a separate measuring system. This possibly causes an error when connection of DUT is changed from the gain measuring system to the noise measuring system. For this background reason, the conventional noise factor measuring system fails to have accurate measurement and evaluation, where the DUT has a small available gain.